Electric wave amplifying system



1939. R. s. CARUTHERS ELECTRIC WAVE AMPLIFYING' SYSTEM Filed Dec. 5,1956 FIG. 3

. /NVEN7 Ok 2 R. s. CARUTHERS FIG. 5

- ATTORNEY Patented July 25, 1939 UNITED STATES PATENT OFFIQE ELECTRICWAVE AMPLIFYING SYSTEM York Application December 5, 1936, Serial No.114,409

Claims.

This invention relates to wave amplifying systems.

An object of the invention is to control transmission properties of suchsystems, as for example, transmission efficiency, distortion,longitudinal currents and impedances in such systems.

In one specific aspect the invention is an amplifier with feedbackrendering the amplifier transparent as to impedance so that theamplifier input impedance is determined by and proportional to the loadimpedance and the amplifier output impedance is determined by andproportional to the impedance of the incoming line or circuit. Thus theamplifier can readily be connected in a circuit such as a filter or longtransmission line without disturbing the impedance conditions.

In another aspect the invention is an amplifier with feedback renderingthe insertion gain of the amplifier independent of frequency. Thus, theamplifier canreadily be inserted in a circuit without changing the shapeof the transmission-frequency characteristic of the circuit.

Another aspect of the invention is an amplifier with feedback renderingits insertion gain for transmission in one direction equal to its lossfor transmission in the opposite direction.

The feedback may be, for example, series negative feedback proportionalto the load voltage and s lunt negative feedback proportional to theload current, such feedbacks also reducing distortion in the amplifierand stabilizing the amplifier gain. The feedback may be provided, forinstance, by a bridge transformer network connecting the incomingcircuit and the amplifier output circuit'each in transmitting relationto the amplifier input circuit and the load circuit, with the ampiifieroutput and input circuits conjugate respectively to the incoming circuitand. the load circuit.

Other objects and aspects of the invention will be apparent from thefollowing description and claims:

Fig. l of the drawing shows a circuit embodying a form of the invention;

Fig. 2 is a circuit diagram facilitating explanation of the operation ofthe circuit of Fig. 1; and

Figs. 3, 4, 5 and 6 show four circuits that are modifications of thecircuit of Fig. 1.

In Fig. 1 is shown a system embodying a form of the invention,comprising an amplifier whose amplifying path may be, for example, anamplifying element of the vacuum tube type having a single vacuum tubestage or any desired number of tandem connected stages, G and Pdesignating the grid of the first tube and the plate of the last tube.

The amplifier may be, for example, a stabilized feedback amplifier ofthe general type in which a portion of the output Wave is fed back ingainreducing phase and in amount sufficient to reduce distortion belowthe distortion level without feedback.

Such feedback is disclosed, for example, in

H. S. Black Patent No. 2,102,671, December 21,-

1937, for Wave translation system, and in the article by Black onStabilized feedback amplifiers, published in Electrical Engineering,January 1934, pages 114 to 120.

The amplifier of Fig. 1 has an input trans- 1 former 2 with a primarywinding m1 of 1m turns and a secondary Winding ms of ms turns, and hasan output transformer 5 with a primary winding m of in turns and a.secondary winding m of n; turns.

Waves to be amplified by the amplifier, as for example, speech currentwaves or speech modulated carrier waves, are supplied from an incomingline or circuit Z1 of impedance value Z1 comprising a wave source E ofvoltage E. The amplified waves are transmitted to the outgoing line orcircuit Z2 of impedance value Z2.

The amplifier comprises a feedback coil m2 of mg turns on transformer 2.This coil m2 is in series with winding m and provides shunt-seriesnegative feedback, or in other Words shunt negative feedbackproportional to the load current. The amplifier also comprises afeedback coil m of m turns on transformer 5. This coil n2 is in serieswith Winding m1 and provides seriesshunt negative feedback, or in otherwords series negative feedback proportional to the load voltage. AC-battery 8 for providing negative grid bias is shown. -A B-battery 9 isshown for supplying space current.

The amplifying path of the amplifier may be referred to as the pcircuit, and the feedback circuits may be referred to as the p circuit,or the 51 circuit in the case of the feedback path through the feedbackcoil m2 and the [32 circuit in the case of the feedback path through thecoil m, the significance of LL and n being as indicated in the patentand article mentioned above. The feedback through the coil m2 may be,for example, negative feedback with .51 much greater than one; and thefeedbacks through the coil n2 may be, for example, negative feedbackwith pflz much greater than one.

The two three-winding transformers 2 and 5 are connected to operate asbiconjugate circuit or bridge transformer network I0 or in other wordsto form a hybrid coil Ill. The hybrid coil l0 connects Z1 and theamplifier output or plate circuit each in transmitting relation to theamplifier input or grid circuit and the load circuit Z2, with theamplifier output and input circuits conjugate respectively to Z1 and Z2.

Transmission from Z1 arriving at the network I0 is partly transmittedthrough windings m and m to Z2 and partly transmitted through windingsm1 and ms to the grid circuit of tube I. Transmission from the platecircuit of the tube is partly transmitted through windings 111 and m toZ2, partly fed through windings m2 and ms back to the grid circuit ofthe tube, and partly fed through windings 721 and m and windings m1 andms back to the grid circuit.

It can readily be seen that the negative feedback action causes the fluxset up by the turns mg to oppose the flux set up by the turns m1.Stability will be reached when the diiferential flux from windings m1and ma produce a voltage in winding m: on the grid of the tube whichwhen amplified to the plate circuit causes a current is to flow in m2 ofthe magnitude necessary for the stabilized value of the differentialflux.

For large gain between grid of the first tube and plate of the lasttube, stability will be reached with the voltage across the gridrelatively zero. The voltages across m1 and ma are consequently zero, orin other words i3mz=i1m1, and

e me C.)

is made equal to unity, it follows that ZA=Z2,

that is, the input impedance equals the load impedance, and ZB=Z1, orthe output impedance equals the incoming line impedance and It isapparent here that the insertion of the feedback amplifier devicebetween Z1 and Z2 results in an amplified picture of the current thatwould flow out of Z1 into Z2 with these two impedances directlyconnected together. This means that the amplifier is transparent as toimpedance.

It is also apparent that a current traversing the circuit in the reversedirection to that assumed in Fig. 1, i. e., flowing out of Z2 into ZB,will encounter a loss equal to the gain of the device in the assumedpositive direction. This may be viewed as a result of symmetry, but in abroader sense, it is a fundamental characteristic of transparentamplifiers.

A further characteristic of the circuit shown in Fig. 1 is that theturns ratios may be so chosen that ZA will equal a constant (It) timesZ2 while 213 equals the reciprocal of this constant times Z1. The gainintroduced by the device is at the same time inedependent of the turnsratios assigned to give the desired impedance values. This point isillustrated by Equations 3 and 4. With the assumption that it is desiredthat ZA=IcZ2 kE I12 .+1 z2 The gain on a power basis is It can readilybe seen that in none of the foregoing equations does m3 appear. Thestabilized performance of the amplifying system is independent of theturns on ma, as a is directly proportional to m3 and ,8 independent ofit.

In Fig. 2, the impedances and conjugacies existing in the stabilizedamplifier are shown. Zg and Zp represent the stabilized grid and platecircuit impedances respectively. The generator in series with Z is thestabilized magnitude of the plate circuit voltage. It can be easilyproven in this diagram that the input circuit of the amplifier isconjugate to the load circuit and that the source is conjugate to theoutput circuit of the amplifier. As indicating in a simple way why, inFig. 1, a source of E. M. F. inserted in the load impedance Z2, forexample, causes substantially no energy to be transmitted to the inputimpedance Zg, it may be noted that (1) without the feedback, since Zp islarge the voltage induced in 11.1 by m produces little current flow inm2, and the voltage induced in the circuit n2, Z1, ml by m would appearprincipally across m1 (since the impedance attached to ms is high), but(2) the negative feedback current through me opposes the flux created in2 by M4, causing the voltage across ml to decrease and the voltageacross 112 to increase, and with high amplification in the tube, at thecondition at which the operation becomes stabilized the voltages acrossm1 and mz are substantially zero, so the voltage transmitted to Zg isvery small.

It is often desired that an amplifier work on a matched impedance basisbetween two impedances Z1 and Z2, have a fixed gain, have a flatgain-frequency characteristic, and have good longitudinal balance (lowmetallic current per volt applied longitudinally). The amplifier cirouitshown can readily be made to satisfy all of these requirements. Forexample, if a 20 decibel gain amplifier is desired with Z1 an impedanceof ohms and Z2 an impedance of. 600 ohms,

and

can be made equal to Then ZA will equal and will match Z1, and Z3 willequal 421 and will match Z2. Not only will the circuit satisfy therequirements noted above, but all transmission performance requirementswill be either improved or stabilized by the negative feedback action.

As indicated above, in an amplifier circuit such as that shown, thenegative feedback can render the insertion gain of the amplifierindependent of frequency, so that, for example, even if Z2 be reactiveor include reactance, the insertion of the amplifier will change thetransmission efficiency of the circuit, for transmission between Z1 andZ2, by the same amount at all frequencies, or in other words, theamplifier can be inserted without changing the shape of thetransmissionfrequency characteristic of the circuit.

In Fig. 3 to Fig. 6, inclusive, circuits are shown that perform in asimilar manner to the amplifier in Fig. l. The circuit of Fig. 3 likethat of Fig. 1 is adapted to working between balanced to groundimpedances of arbitrary impedance ratioand with arbitrary gain.

Figs. i, 5 and 6 show modifications of the circuit of Fig. 1 adapted foroperation only between unlike impedances. Figs. 4 and 5 are circuitsadapted for grounded circuit operation. Fig. 6 is suitable forbalanced-to-ground operation.

What is claimed is:

l. A wave translating system comprising a wave amplifying device havinginput and output impedances, and means feeding back in said device wavesthat cause said input and output impedances to be determined by andproportional to the receiving and sending impedances, respectively,connected to said input and output impedances, said device comprising anamplifying element having input and output circuits and said meanscomprising an impedance in shunt relation to said input circuit withrespect to said sending impedance and in serial relation to thereceiving impedance with respect to said output circuit, and animpedance in serial relation to said input circuit with respect to thesending impedance and in shunt relation to the receiving impedance withrespect to said output circuit.

2. A wave translating system comprising a wave amplifying device havinginput and output impedances, sending and receiving impedances attachedto said input and output impedances, respectively, and subject tovariations, and means producing negative feedback in said device ofwaves that cause said input and output impedances to maintain fixedratios to said varying receiving and sending impedances, respectively,said device comprising an amplifying element having input and outputcircuits and said means comprising means for producing shunt feedback tosaid input circuit proportional to current flowing in said outputcircuit and series feedback to said input circuit proportional to thevoltage across said receiving impedance.

3. A system comprising wave amplifying means and a load impedancetherefor variable with frequency, said amplifying means comprising anamplifying device and a network feeding waves back in said device ingain-reducing phase and rendering the input impedance of said amplifyingmeans equal to av constant times said load impedance, said networkcomprising input and output transformers for said amplifying devicehaving primary and secondary windings, a feedback coil in series withthe primary winding of said input transformer and inductively related tosaid windings of said output transformer, and a feedback coil in serieswith the primary winding of, said output transformer and inductivelyrelated to said windings of said input transformer.

4. A wave translating system comprising wave amplifying means, a sendingimpedance therefor variable with frequency, said amplifying meanscomprising an amplifying device and a network for feeding waves back insaid device in gainreducing phase and rendering the output impedance ofsaid amplifying means equal to a constant times said sending impedance,said network comprising input and output transformers for saidamplifying device having primary and secondary windings, a feedback coilin series with the primary winding of said input transformers andinductively related to said windings of. said output transformer, and afeedback coil in series with the primary winding of said outputtransformer and inductively related to said windings of said inputtransformer.

5. A wave translating system comprising a wave amplifying device, twowave transmission circuits, and a network interconnecting said deviceand said circuits and producing negative feedback in said device ofwaves that cause the impedance of one of said circuits to appear as theimpedance facing the other circuit, said network comprising input andoutput transformers for said amplifying device having primary andsecondary windings, a feedback coil in series with;

one of said windings of. said output transformer and inductively relatedto said windings of said input transformer, and a feedback coil inseries with one of the two first-mentioned windings of said inputtransformer and inductively related to said windings of said outputtransformer.

6. A wave amplifying device having input and output circuits, a wavesource for association with said input circuit, a load circuit forassociation with said output circuit, and means for producinggain-reducing feedback in said device, said means comprising animpedance in shunt relation to said input circuit with respect to saidsource and in serial relation to said load circuit with respect to saidoutput circuit, and an impedance in serial relation to said inputcircuit with respect to said source and in shunt relation to said loadcircuit with respect to said output circuit.

'7. A wave implifying system having means for producing shunt feedbackto the input of the system proportional to current flowing in the loadimpedance and series feedback to the input of the system proportional tothe voltage across the load impedance.

8. A wave transmission system comprising a wave amplifying circuit andattached incoming and outgoing circuits therefor, one of said attachedcircuits comprising reactance, and said amplifying circuit having meansfeeding back therein waves that reduce its gain and render its insertiongain independent of frequency, said means producing shunt feedback tothe input of said amplifying circuit proportional to the amplifiedoutput current of said amplifying circuit and series feedback to theinput of said amplifying circuit proportional to the voltage across theload impedance.

9. A wave translating system comprising a wave amplifying device,incoming and outgoing circuits, and a network comprising twothreewinding transformers interconnecting said de vice and said circuitsand producing negative feedback in said device of waves that cause theimpedance of said network facing one of said two circuits to equal aconstant times the impedance of the other of said two circuits and causethe impedance of said network facing said other circuit to equal aconstant times the impedance of said one circuit, one of saidtransformers having primary and secondary windings connectedrespectively to said incoming circuit andsaid device, the other of saidtransformers having primary and secondary windings connectedrespectively to said device and said outgoing circuit, said onetransformer having a feedback winding inductively related to its primaryand secondary windings and carrying a feedback current proportional tothe current in one of said windings of. said other transformer, and saidother transformer having a feedback winding inductively related to itsprimary and secondary windings and carrying a feedback currentproportional to the current in one of the two first-mentioned windingsof said one transformer.

10. A wave translating system comprising a wave amplifying device havingan input circuit and an output transformer with a primary winding and asecondary winding for coupling said device to a load impedance, meansfor producing negative feedback in said device comprising a feedbackimpedance in shunt relation to said input circuit and in serial relationto said primary winding of said output transformer, and means forproducing series negative feedback in said device proportional to thevoltage across the load impedance.

primary winding of said transformer for producing negative feedback insaid amplifier proportional to the voltage across the load impedance,and means for producing shunt negative feedback in said amplifierproportional to the current in the load impedance.

12. An amplifier with input and output transformers each having aprimary winding and a secondary winding, a feedback coil in serialrelation to said primary winding of said output transformer andinductively related to said windings of said input transformer, and afeedback coil in serial relation to said primary winding of said inputtransformer and inductively related to said windings of said outputtransformer.

13. A system comprising wave amplifying means having an input circuitand an output circuit, a wave source and a load circuit, and a networkconnecting said load circuit for transmission to said source andconnecting said output circuit for transmitting to said load circuit,said network having inductively related branches which are connectedrespectively to said source and said output circuit in opposing relationwith respect to said input circuit and which substantrtlly preventtransmission from said load circuit to said input circuit.

14. A wave translating system comprising a wave amplifying device andmeans for feeding back in said device waves that render the insertiongain introduced in traversing said system in one direction equal to theloss introduced in traversing said system in the opposite direction,said means comprising input and output transformers for said amplifyingdevice having primary and secondary windings, a feedback coilinductively related to said windings of said input transformer andcarrying current proportional to the load current of the amplifier, anda second feedback coil inductively related to said windings of saidoutput transformer and carrying current proportional to that in one ofthe two firstmentioned windings of said input transformer.

15. An amplifier with an input transformer and an output transformereach having a primary winding and a secondary winding, and means forproducing negative feedback in said amplifier comprising a feedback coilinductively related to said windings of said input transformer andcarrying a current proportional to the current in one of said windingsof said output transformer, and a second feedback coil inductivelyrelated to said windings of said output transformer and carrying acurrent proportional to the current flowing in one of the windings ofsaid input transformer.

ROBERT S. CARUTI-IERS.

